WO-A-0079092 discloses such drilling system, whereby the discharge means control the discharge of drilling fluid,and therewith the flow of drilling fluid through the annular space. Therefore the discharge conduit is provided with a controllable outlet valve. As an alternative WO-A-0079092 describes an injection pump arranged to pump injection fluid via an injection nozzle into the discharge conduit in a direction opposite to the direction of flow of drilling fluid through the discharge conduit. By controlling the injection fluid, the resistance in the discharge conduit can be controlled.